Any computer system operating at a given clock frequency generates electromagnetic radiation of a frequency related to the clock frequency. Legislative and industry practice standards define maximum levels for these emissions so as to keep them within safe levels. Examples of formal tests of electromagnetic radiation in these situations are NEBs, FCC and European CE mark, and traditionally the intensity of radiation is measured by instruments with bandwidths of about 120 KHz.
When several CPUs are located in close proximity to each other, for example when multiple cores are involved, then their emissions tend to be cumulative and the additive effect of the emissions, particularly in a large concentration of CPUs, may inadvertently exceed legal standards at certain frequencies, even though emissions from each individual core meet the standard. Typically the higher emissions will be at harmonic frequencies or sub-harmonics of the CPU's clock and memory bus frequency (dependent on the number of CPUs).
Previously this problem has been addressed by a spread spectrum technique in which the memory bus clock on a single server is continually varied across a predefined range. Such technology is built into a chip so that it is effectively integral with the CPU and it is thus relatively complex and cannot be retrofitted or easily altered or adapted.